Kerberos Working Group Karthik Jaganathan
Internet Draft Larry Zhu
Document: draft-ietf-krb-wg-kerberos-referrals-04.txt John Brezak
Category: Standards Track Microsoft
Mike Swift
University of Washington
Jonathan Trostle
Cisco Systems
Expires: January 2005
Generating KDC Referrals to locate Kerberos realms
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC-2026 [1].
Internet-Drafts are working documents of the Internet Engineering
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Abstract
The draft documents a method for a Kerberos Key Distribution Center
(KDC) to respond to client requests for Kerberos tickets when the
client does not have detailed configuration information on the realms
of users or services. The KDC will handle requests for principals in
other realms by returning either a referral error or a cross-realm
TGT to another realm on the referral path. The clients will use this
referral information to reach the realm of the target principal and
then receive the ticket.
Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [2].
1. IntroductionJaganathan [Page 1]

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Current implementations of the Kerberos AS and TGS protocols, as
defined in [3], use principal names constructed from a known user or
service name and realm. A service name is typically constructed from
a name of the service and the DNS host name of the computer that is
providing the service. Many existing deployments of Kerberos use a
single Kerberos realm where all users and services would be using the
same realm. However in an environment where there are multiple
trusted Kerberos realms, the client needs to be able to determine
what realm a particular user or service is in before making an AS or
TGS request. Traditionally this requires client configuration to make
this possible.
When having to deal with multiple trusted realms, users are forced to
know what realm they are in before they can obtain a ticket granting
ticket (TGT) with an AS request. However, in many cases the user
would like to use a more familiar name that is not directly related
to the realm of their Kerberos principal name. A good example of this
is an RFC-821 style email name [4]. This document describes a
mechanism that would allow a user to specify a user principal name
that is an alias for the user's Kerberos principal name. In practice
this would be the name that the user specifies to obtain a TGT from a
Kerberos KDC. The user principal name no longer has a direct
relationship with the Kerberos principal or realm. Thus the
administrator is able to move the user's principal to other realms
without the user having to know that it happened.
Once a user has a TGT, they would like to be able to access services
in any trusted Kerberos realm. To do this requires that the client be
able to determine what realm the target service principal is in
before making the TGS request. Current implementations of Kerberos
typically have a table that maps DNS host names to corresponding
Kerberos realms. In order for this to work on the client, each
application canonicalizes the host name of the service, for example
by doing a DNS lookup followed by a reverse lookup using the returned
IP address. The returned primary host name is then used in the
construction of the principal name for the target service. In order
for the correct realm to be added for the target host, the mapping
table [domain_to_realm] is consulted for the realm corresponding to
the DNS host name. The corresponding realm is then used to complete
the target service principal name.
This traditional mechanism requires that each client have very
detailed configuration information about the hosts that are providing
services and their corresponding realms. Having client side
configuration information can be very costly from an administration
point of view - especially if there are many realms and computers in
the environment.
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There are also cases where specific DNS aliases (local names) have
been setup in an organization to refer to a server in another
organization (remote server). The server has different DNS names in
each organization and each organization has a Kerberos realm that is
configured to service DNS names within that organization. Ideally
users are able to authenticate to the server in the other
organization using the local server name. This would mean that the
local realm be able to produce a ticket to the remote server under
its name. You could give that remote server an identity in the local
realm and then have that remote server maintain a separate secret for
each alias it is known as. Alternatively you could arrange to have
the local realm issue a referral to the remote realm and notify the
requesting client of the server's remote name that should be used in
order to request a ticket.
This draft proposes a solution for these problems and simplifies
administration by minimizing the configuration information needed on
each computer using Kerberos. Specifically it describes a mechanism
to allow the KDC to handle canonicalization of names, provide for
principal aliases for users and services and provide a mechanism for
the KDC to determine the trusted realm authentication path by being
able to generate referrals to other realms in order to locate
principals.
To rectify these problems, this draft introduces three new kinds of
KDC referrals:
1. AS ticket referrals, in which the client doesn't know which realm
contains a user account.
2. TGS ticket referrals, in which the client doesn't know which realm
contains a server account.
3. Cross realm shortcut referrals, in which the KDC chooses the next
path on a referral chain
2. Requesting a referral
In order to request referrals defined in section 5, 6, and 7, the
Kerberos client MUST explicitly request the canonicalize KDC option
(bit 15) [3] for the AS-REQ or TGS-REQ. This flag indicates to the
KDC that the client is prepared to receive a reply that contains a
principal name other than the one requested.
KDCOptions ::= KerberosFlags
-- canonicalize (15)
-- other KDCOptions values omitted
The client should expect, when sending names with the "canonicalize"
KDC option, that names in the KDC's reply MAY be different than the
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name in the request. A referral ticket is a cross realm TGT that is
returned when the sname of the ticket is not the sname in the request
[3].
3. Realm Organization Model
This draft assumes that the world of principals is arranged on
multiple levels: the realm, the enterprise, and the world. A KDC may
issue tickets for any principal in its realm or cross-realm tickets
for realms with which it has a direct trust relationship. The KDC
also has access to a trusted name service that can resolve any name
from within its enterprise into a realm. This trusted name service
removes the need to use an un-trusted DNS lookup for name resolution.
For example, consider the following configuration, where lines
indicate trust relationships:
MS.COM
/ \
/ \
OFFICE.MS.COM NTDEV.MS.COM
In this configuration, all users in the MS.COM enterprise could have
a principal name such as alice@MS.COM, with the same realm portion.
In addition, servers at MS.COM should be able to have DNS host names
from any DNS domain independent of what Kerberos realm their
principals reside in.
4. Client Name Canonicalization
A client account may have multiple principal names. More useful,
though, is a globally unique name that allows unification of email
and security principal names. For example, all users at MS may have a
client principal name of the form "joe@MS.COM" even though the
principals are contained in multiple realms. This global name is
again an alias for the true client principal name, which indicates
what realm contains the principal. Thus, accounts "alice" in the
realm NTDEV.MS.COM and "bob" in OFFICE.MS.COM may logon as
"alice@MS.COM" and "bob@MS.COM".
This utilizes a new client principal name type, as the AS-REQ message
only contains a single realm field, and the realm portion of this
name doesn't correspond to any Kerberos realm. Thus, the entire name
"alice@MS.COM" is transmitted as a single component in the client
name field of the AS-REQ message, with a name type of NT-ENTERPRISE
[3]. The KDC will recognize this name type and then transform the
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requested name into the true principal name. The true principal name
can be using a name type different from the requested name type.
Typically the true principal name will be a NT-PRINCIPAL [3].
If the "canonicalize" KDC option is set, then the KDC MAY change the
client principal name and type in the AS response and ticket returned
from the name type of the client name in the request. For example the
AS request may specify a client name of "bob@MS.COM" as an NT-
PRINCIPAL with the "canonicalize" KDC option set and the KDC will
return with a client name of "104567" as a NT-UID.
5. Client Referrals
The simplest form of ticket referral is for a user requesting a
ticket using an AS-REQ. In this case, the client machine will send
the AS-REQ to a convenient trusted realm, for example the realm of
the client machine. In the case of the name alice@MS.COM, the client
MAY optimistically choose to send the request to MS.COM. The realm in
the AS-REQ is always the name of the realm that the request is for as
specified in [3].
The KDC will try to lookup the name in its local account database. If
the account is present in the realm of the request, it SHOULD return
a KDC reply structure with the appropriate ticket.
If the account is not present in the realm specified in the request
and the "canonicalize" KDC option is set, the KDC will try to lookup
the entire name, alice@MS.COM, using a name service. If this lookup
is unsuccessful, it MUST return the error KDC_ERR_C_PRINCIPAL_UNKNOWN
[3]. If the lookup is successful, it MUST return an error
KDC_ERR_WRONG_REALM [3] and in the error message the crealm field
will contain either the true realm of the client or another realm
that MAY have better information about the client's true realm. The
client MUST NOT use a cname returned from a referral.
If the client receives a KDC_ERR_WRONG_REALM error, it will issue a
new AS request with the same client principal name used to generate
the first referral to the realm specified by the realm field of the
Kerberos error message from the first request. The client SHOULD
repeat these steps until it finds the true realm of the client. To
avoid infinite referral loops, an implementation should limit the
number of referrals. A suggested limit is 5 referrals before giving
up. In Microsoft's current implementation through the use of global
catalogs any domain in one forest is reachable from any other domain
in the same forest or another trusted forest with 3 or less
referrals.
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The primary problem in service referrals is that the KDC must return
a referral ticket rather than an error message as is done in AS
ticket referrals. There needs to be a place to include in the TGS-REP
information about what realm contains the service. This is done by
returning information about the service name in the pre-
authentication data field of the KDC reply [3].
If the KDC resolves the service principal name into a principal in
the realm specified by the service realm name, it will return a
normal ticket.
If the "canonicalize" flag in the KDC options is not set, the KDC
MUST only look up the name as a normal principal name in the
specified service realm. If the "canonicalize" flag in the KDC
options is set and the KDC doesn't find the principal locally, the
KDC MAY return a cross-realm ticket granting ticket to the next hop
on the trust path towards a realm that may be able to resolve the
principal name.
When a referral TGT is returned, the KDC MUST return the target realm
for the referral TGT as an KDC supplied pre-authentication data
element in the response. The pre-authentication data MUST be
encrypted using the sub-session key from the authenticator if present
or the session key from the ticket. The pre-authentication data
contains the referred realm, and if known, the real principal name.
PA-SERVER-REFERRAL 25
PA-SERVER-REFERRAL-DATA ::= EncryptedData
-- ServerReferalData --
ServerReferralData ::= SEQUENCE {
referred-realm [0] Realm,
-- target realm of the referral TGT
referred-name [1] PrincipalName OPTIONAL,
-- service principal name, MAY differ
-- from the server name in the request
...
}
Clients MUST NOT process referral tickets if the KDC response does
not contain the PA-SERVER-REFERRAL.
If applicable to the encryption type, the key usage value for the
encryption key used by PA-SERVER-REFERRAL is 26 if the session key
from the ticket is used or 27 if a sub-session key is used.
If the referred-name field is present, the client MUST use that name
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in a subsequent TGS request to the service realm when following the
referral.
The client will use this information to request a chain of cross-
realm ticket granting tickets until it reaches the realm of the
service, and can then expect to receive a valid service ticket.
However an implementation should limit the number of referrals that
it processes to avoid infinite referral loops. A suggested limit is 5
referrals before giving up.
Here is an example of a client requesting a service ticket for a
service in realm NTDEV.MS.COM where the client is in OFFICE.MS.COM.
+NC = Canonicalize KDCOption set
+PA-REFERRAL = returned PA-SERVER-REFERRAL
C: TGS-REQ sname=server/foo.ntdev.ms.com +NC to OFFICE.MS.COM
S: TGS-REP sname=krbtgt/MS.COM@OFFICE.MS.COM +PA-REFERRAL
containing MS.COM as the referred realm with no referred name
C: TGS-REQ sname=server/foo.ntdev.ms.com +NC to MS.COM
S: TGS-REP sname=krbtgt/NTDEV.MS.COM@MS.COM +PA-REFERRAL
containing NTDEV.MS.COM as the referred realm with no referred
name
C: TGS-REQ sname=server/foo.ntdev.ms.com +NC to NTDEV.MS.COM
S: TGS-REP sname=server/foo.ntdev.ms.com@NTDEV.MS.COM
7. Cross Realm Routing
The current Kerberos protocol requires the client to explicitly
request a cross-realm TGT for each pair of realms on a referral
chain. As a result, the client need to be aware of the trust
hierarchy and of any short-cut trusts (those that aren't parent-
child trusts).
Instead, using this referral routing mechanism, The KDC will
determine the best path for the client and return a cross-realm TGT
as the referral TGT, and the target realm for this TGT in the PA-
SERVER-REFERRAL of the KDC reply.
If the "canonicalize" KDC option is not set, the KDC MUST NOT return
a referral ticket. Clients MUST NOT process referral tickets if the
KDC response does not contain the PA-SERVER-REFERRAL.
8. Compatibility with earlier implementations of name canonicalization
The Microsoft Windows 2000 and Windows 2003 releases included an
earlier form of name-canonicalization [5]. Here are the differences:
1) The TGS referral data is returned inside of the KDC message as
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"encrypted pre-authentication data".
EncKDCRepPart ::= SEQUENCE {
key [0] EncryptionKey,
last-req [1] LastReq,
nonce [2] UInt32,
key-expiration [3] KerberosTime OPTIONAL,
flags [4] TicketFlags,
authtime [5] KerberosTime,
starttime [6] KerberosTime OPTIONAL,
endtime [7] KerberosTime,
renew-till [8] KerberosTime OPTIONAL,
srealm [9] Realm,
sname [10] PrincipalName,
caddr [11] HostAddresses OPTIONAL,
encrypted-pa-data [12] SEQUENCE OF PA-DATA OPTIONAL
}
2) The preauth data type definition in the encrypted preauth data is
as follows:
PA-SVR-REFERRAL-INFO 20
PA-SVR-REFERRAL-DATA ::= SEQUENCE {
referred-name [1] PrincipalName OPTIONAL,
referred-realm [0] Realm
}
9. Security Considerations
In the case of TGS requests the client may be vulnerable to a denial
of service attack by an attacker that replays replies from previous
requests. The client can verify that the request was one of its own
by checking the client-address field or authtime field, though, so
the damage is limited and detectable.
For the AS exchange case, it is important that the logon mechanism
not trust a name that has not been used to authenticate the user.
For example, the name that the user enters as part of a logon
exchange may not be the name that the user authenticates as, given
that the KDC_ERR_WRONG_REALM error may have been returned. The
relevant Kerberos naming information for logon (if any), is the
client name and client realm in the service ticket targeted at the
workstation that was obtained using the user's initial TGT.
How the client name and client realm is mapped into a local account
for logon is a local matter, but the client logon mechanism MUST use
additional information such as the client realm and/or authorization
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